It is generally accepted, that increasing oil prices force tire and rubber producers to contribute to the production of “fuel efficient” and thus fuel or gas saving tires. One general approach to obtain less fuel consuming tires consists in the reduction of the hysteresis loss. The hysteresis loss of a cross-linked elastomeric polymer composition is related to its Tan δ value at 60° C. (see ISO 4664-1:2005; Rubber, Vulcanized or thermoplastic; Determination of dynamic properties—part 1: General guidance). In general, vulcanized elastomeric polymer compositions having relatively small Tan δ values at 60° C. are preferred as having lower hysteresis loss. In tires, this translates to a lower rolling resistance and better fuel economy. One method to reducing hysteresis loss is to modify chain ends of uncrosslinked elastomeric polymers. Various techniques are described in the open literature including the use of “coupling agents” such as tin tetrachloride, which may functionalize the polymer chain end, and react with components of an elastomeric composition, such as for example with a filler. Examples of such techniques along with other documents of interest include: U.S. Pat. Nos. 3,281,383; 3,244,664 and 3,692,874 (for example tetrachlorosilane); U.S. Pat. Nos. 3,978,103; 4,048,206; 4,474,908; 6,777,569 (blocked mercaptosilanes) and U.S. Pat. No. 3,078,254 (a multi-halogen-substituted hydrocarbon such as 1,3,5-tri(bromo methyl)benzene); U.S. Pat. No. 4,616,069 (tin compound and organic amino or amine compound) and U.S. Publication 2005/0124740.
“Synthesis of end-functionalized polymer by means of living anionic polymerization” Journal of Macromolecular Chemistry and Physics 197 (1996), 3135-3148, describes the synthesis of polystyrene-containing and polyisoprene-containing living polymers with hydroxy (—OH) and mercapto (—SH) functional end caps, obtained by reacting the living polymer with haloalkanes containing silyl ether and silyl thioether functions. The tertiary-butyldimethylsilyl (TBDMS) group is preferred as protecting group for the —OH and —SH functions in the termination reactions, because the corresponding silyl ethers and thioethers are found to be both, stable and compatible with anionic living polymers.
Another method of reducing the hysteresis loss consists in a statistical backbone functionalization of polydienes. The number of functionalized positions along the rubber backbone can be higher compared with chain-end modification, where just one or two positions of one base polymer chain can be modified. Accordingly the interaction of the polymer chain with filler particles can be more intense in case of a backbone modification, when multiple (more than two) polymer backbone positions are modified. Thus, backbone functionalized polydienes have a potential for a lower hysteresis loss and a better compatibility with fillers, such as with carbon black and silica.
Examples for backbone modification of polymers.
Some typical examples for a polymer backbone modification were reported.
The Examples of such technique include:    (1) U.S. Pat. No. 6,933,358, and    (2) U.S. Pat. No. 7,041,761.
Polydienes prepared by using an anionic polymerization technology are usually made from butadiene, isoprene, and optionally styrene, as monomer sources. Functionalized conjugated diene or vinylaromatic monomers can be incorporated into the growing polymer chain, when interactions of the functional group with the initiator compound, often represented by n-butyllithium or sec-butyllithium, are avoided. Functionalized monomers, representing substituted styrene (1, 2) are shown below.

In the above structures, (R and R′ are hydrogen or an alkyl group; x is the number 1 to 10; m and n are the numbers 0 to 10). Functionalized monomers representing, substituted isoprene (1) are shown below.

The substituted styrene and substituted isoprene structures, as shown above, were incorporated into the elastomeric polydiolefin by being copolymerized with unfunctionalized conjugated diolefin, and optionally vinyl aromatic monomers. The functionalized polymers improved the compatibility of the rubber in tires with fillers, such as carbon black and silica. In particular, tire rolling resistance properties, and therefore hysteresis properties of tires were stated to be improved. For example 0.25, 1, 2 and 5 weight percentage of 4-(2-pyrrolidinoethyl)styrene (PES) or 3-(pyrrolidino-methyl-2-ethyl) α-methyl-styrene were incorporated into a butadiene-styrene backbone. The resulting functionalized polymer was applied to the preparation of silica-rubber compounds, and the rolling resistance related tan delta values were measured at 60° C. The reference polymer sample not containing any backbone functionalization resulted in a tan delta value of 0.145, while the highest modification degree (5 wt %) led to a decreased tan delta value of 0.073.
Polar groups containing compounds, called, and defined as, polar coordinator compounds in the present disclosure, however are known to influence the polymerization kinetic, and thus the 1,2-polybutadiene, as well as the styrene, composition distribution and concentration in the (co)polymer. Therefore the application of functionalized monomers requires the development of new polymerization recipes, when an equivalent microstructure of the functionalized polymer is desired. Accordingly, it is beneficial to modify the polymer backbone after essentially completing the monomer conversion.
There are a few references describing the vulcanization of polydienes with low molecular weight dithiols in the presence of peroxides.
The Examples of such technique include:    (3) Strecker, R. A. H., Rubber Chemistry and Technology, 44/3, (1971), 675-689, and    (4) U.S. Pat. No. 3,876,723.
According to reference (3), liquid polybutadienes were cured with dithiols of lower molecular weight, such as, for example, hexane-1,6-dithiol and 3,4-dimercaptotoluene in the presence of dicumylperoxide or t-butylperbenzoate, as initiator, at temperatures below 100° C. The products of the curing reaction had an improved stability against hydrolysis and air oxidation. Reportedly “strain at break” values decreased with an increasing dithiol concentration. No cure was obtained with the peroxide absent. Similarly, U.S. Pat. No. 3,876,723 (4) describes a method of curing a polydiene using a polydithiol, a polyepoxide and an amine compound (the last two compounds representing a radical initiator system) in a period of 50 to 84 hours at a temperature of 80 to 100° C. As result of the dithiol addition, increased vulcanizate elongation values were reported. Thus, both references describe peroxide initiated reactions of the dithiol with the polydiene.
For a backbone modification of a polydiene, prepared in solution, it would be beneficial to avoid the addition of peroxide initiators. The peroxide represents an additional component in the polymerization process, which also causes polymer chain crosslinking through radical formation along the polymer backbone. In addition the peroxide initiated functionalization requires extra time. Also side products of the peroxide initiator and of the amine component need to be considered, and in some occasions need to be removed. Therefore, it would be beneficial to react a thiol or sulfide containing compound with the polymer backbone, without having the need of auxiliary compounds.
Other references describe the curing of low molecular weight polybutadiene in the presence of low molecular weight dithiols.
The Examples of such technique include:    (5) U.S. Pat. No. 3,338,810, and    (6) U.S. Pat. No. 2,964,502.
A method of making a clear solid polymer comprising reacting a low molecular weight (C2-C6) alkyl polythiol Y(SH)n (n=2-6), and a liquid polydiene, in the presence of ultraviolet light, is described in reference (5). The molecular weight of liquid polydienes usually is considered to be below 50,000 g/mol, where it becomes waxy. The molecular weight of 1500 g/mol, given as example, is in the proposed range for a liquid diene polymer. A solid polymer is, according to the given examples, obtained after 15 or 30 hours respectively. In another reference (5) “in-situ” made 2,5-dimercapto-1,3,4-thiadiazole was reacted with liquid polybutadiene to produce a solid polymer product. The curing reaction was initiated by heating the mixture up to a temperature of, for example, 188° C. This temperature is needed for a relatively long period, such as exemplary, 73 hours.
The curing period as presented in reference (5) and (6) is very long. Compound vulcanizates prepared for tires need to be prepared in minutes, such as, for example, within 5 to 30 minutes. Accordingly such a long vulcanization period would not be acceptable for the production of elastomeric tire components. In addition liquid polymers can not be easily processed by a tire producer.
Additionally, vulcanizates are reportedly made from low molecular weight dithiol containing polydiene carbon black compounds. The dithiols were added in the course of the compounding or milling process.
Examples of such techniques include:
    (7) Hull C. M. et al., Journal of Industrial and Engineering Chemistry, 40, (1948), 513-517.
In reference (7), the characteristic of emulsion butadiene-styrene rubber (GR-S rubber)—carbon black compound vulcanizates, prepared in the presence of low molecular weight dithiols was investigated. Modulus 300%, and tensile strength, of the compound vulcanizates changed when different dithiols were added. The tensile strength of vulcanizates made by using dithiols was mentioned to be higher than the use of one of dithiol free vulcanizates. The addition of dithiol compounds to the GR-S cement resulted in cement gelation, assumed to be caused from enhanced polymer chain cross-linking. Aside from the aforementioned gelation experiment, Table III suggests that the dithiols were always added as pure component to the carbon black compound mixture, prior to the vulcanization. Thus, the low molecular weight dithiols are only combined with an uncrosslinked elastomeric polymer during compounding. This approach is disadvantaged due to the difficulty of distributing the dithiol compound throughout the rubber mixture during compounding. That is, unlike the typical low viscous, solvent-based environment associated with most anionic polymerizations, the rubber compounding environment is typically highly viscous and solvent free, thus leading to a less homogenous distribution of the coupling agent throughout the composition. As consequence, the interaction of the functionalized polymer with the filler material and/or unsaturated segments of the polymer backbone is less balanced and thus less complete.
Another example, U.S. Pat. No. 6,696,523, describes hydroxyl group-containing rubbers built up from diolefins and from hydroxylmercaptanes and mercaptocarboxylic esters in combination with radical starters such as for example azobisisobutyronitrile or dilauroylperoxide. For a backbone modification of a polydiene, it would be beneficial to avoid the addition of radical forming initiator compounds as discussed above.
And yet polyfunctional thiol—monofunctional thiol mixtures were applied as chain transfer agent in the preparation of emulsion made styrene-butadiene latex (8).
Examples of such techniques include: (8) International Publication No. WO 02/50128.
After forming seed latex particles, in a first polymerization step, the polyfunctional thiol was added as component in a second and third step. The second and third polymerization step were performed to cover the seed latex particles with a first and a second shell. The resulting latex, when applied to the preparation of coated paper, was reported to have an improved polymerization stability, mechanical stability and adhesion. The dithiol was exclusively mentioned in the patent application to act as chain transfer agent, and thus the dithiol is not expected to yield a functionalized polymer using this technology.
Furthermore backbone modified polybutadienes of the general formula:
are described in reference (9), German Application No. 1,910,177, for the preparation of HIPS and varnish.
According to the reference (9), the preparation of the modified polybutadienes was performed through reaction of preferably low molecular weight polydienes (Mw<100,000 g/mol) with low molecular weight aminothiols (HS—R—NH2 and HS—R(COOH)—NH2) in the presence of suitable radical forming initiator compounds such as for example azodiisobutyronitril.
As stated above for a backbone modification of a polydiene, prepared in solution, it would be beneficial not to add peroxide initiators to avoid at least one of the following:    a) additional reaction time for a further modification step, and    b) problems with additional components in the polymer preparation process    c) carbon-carbon crosslink formation.
Therefore, it would be beneficial to react a thiol or sulfide containing compound with the polymer backbone, without having the need of auxiliary compounds. Also it would be beneficial to react sufficiently high molecular weight thiol or sulfide containing compounds with the polymer backbone to avoid a partial or complete removal of backbone modification agent prior to polymer modification, during polymer work-up, which often involves elevated work-up process temperatures.
Below described backbone modification mechanism is especially beneficial for high molecular weight polymers though no limitation is given to the molecular weight of polymers. In case of high molecular weight rubber, the proportion by weight of end groups is small and can therefore have only a small effect on the interaction between filler and rubber or between different rubber polymer chains. The present invention is intended to provide polymer chains in elastomeric polymer compositions having a much higher concentration of polymer bound effective groups for interacting with fillers and/or with polymer chains.